John Thomson,2 Ian Jarvis,3
Darryl R.H. Green,2 and Damon Green 3

ABSTRACT

The emplacement of organic-rich turbidite units on
abyssal plains is commonly followed by the early
diagenetic redistribution of redox-sensitive elements.
This occurs only at the summit of the turbidite unit,
where it is caused by the downward diffusion of seawater
oxygen, which continues for as long as each turbidite is
in diffusive contact with bottom waters. The duration of
this process is generally a few tens of thousands of
years on the Madeira Abyssal Plain (MAP), where turbidite
emplacement occurs at frequent intervals. The known
sequence of redox-sensitive element redistribution peaks
around oxidation fronts in turbidites, derived from
studies of conventional piston core material sampling the
upper few tens of meters of Quaternary sediments on the
MAP, is compared here with that in a single turbidite
recovered at Site 950, and now buried to 230 mbsf. This
unit was deposited during the late Miocene (~7.6 Ma) and
has experienced consecutively post-oxic and
sulfate-reducing conditions during burial. The organic
carbon (Corg ) profile and Corg /S
ratio are remarkably similar to those in more recent
examples. The labile (nondetrital) fractions of many
redox-sensitive elements remain concentrated around a
sharp color change from dark green (below) to greenish
gray (above), which marks the limit of penetration of
oxygen when the turbidite top was exposed on the
seafloor. The elements Cd, Sb, Se, Tl, and V are all
still found in peaks immediately below the original
oxic/postoxic boundary although, as in the younger
turbidites, the peak maxima are not exactly coincident.
Cobalt, Cu, Ni, and Zn have apparently moved upward in
chromatographic-like fronts, which are seen as thin
dark-purple or gray bands in the core. Labile As, S, and
U, which were selectively removed from sediment above the
color change by the bacterially driven redox processes
that accompanied the oxic decomposition of Corg
in the turbidite, have retained their characteristic
stepped-concentration pro-files. The successive passage
through ~130 m of post-oxic and ~100 m of
sulfate-reducing conditions (each lasting ~3.8 m.y.) has
had remarkably little effect on trace-metal
distributions.